The best I've seen so far is Prawn:

  • Prawn http://prawn.majesticseacreature.com/
  • http://advent2008.hackruby.com/past/2008/12/17/awn_and_the_future_of_ruby_pdf_generation/
  • http://railstips.org/2008/10/14/how-to-generate-pdfs-in-rails-with-prawn
  • Prawn Rails plugin: http://www.cracklabs.com/prawnto

Prawn with Prawnto for sure. The DSL is a real treat, as is the simplicity of being able to treat PDF as any other format in a respond_to format block:

respond_to do |format|
format.pdf { render :layout => false }

There's a tutorial video on Prawn here: